This invention relates to crosstalk reduction or rejection between signal channels.
In tape recording, low-frequency response problems prevent the recording of broad instantaneous bandwidth materials. For example, recording of composite chrominance and luminance information cannot readily be accomplished. Head-to-tape contact varies with time, and as a consequence video tape recorders use FM modulation to improve signal-to-noise and frequency response. The bandwidth problem remains in such systems, however, and often the luminance information is recorded separately from the chrominance information. In such cases, the luminance is frequency-modulated on a high-frequency carrier for best noise performance, while the less critical chrominance information is directly modulated with the aid of an AC bias signal onto the same track as the luminance information. In order to reduce the interfering beats between the FM luminance carrier and the AC bias signal, the FM carrier may itself be used as the AC bias signal, although the FM carrier amplitude must be reduced from the maximum possible amplitude to avoid saturation of the tape and consequent distortion of the chrominance signal. The directly recorded chrominance signal can occupy only that portion of the frequency spectrum below the lowest deviation frequency of the luminance carrier.
In order to obtain greater chrominance fidelity, increased bandwidth may be desirable. This bandwidth is not available in the track on which the luminance information is recorded, so the chrominance information may be recorded onto a separate adjacent track. On that track, the chrominance information may be directly recorded. Since the desired bandwidth of the chrominance carrier is greater than the bandwidth available below the lowest deviation frequency of the luminance information carrier on the adjacent track, a portion of the chrominance information band of frequencies lies within the band of frequency used for the luminance information. Imprinting from track to track of the tape or coupling between the tracks and adjacent playback heads causes crosstalk between the two signal channels (luminance and chrominance) in the frequency overlap region. Electrical filtration of the signals cannot be used to eliminate the undesired signals occurring in the overlap region because the desired signals would also be eliminated. The unwanted coupling may be reduced by azimuth recording or by providing a guardband between the tracks of the tape as described for example in U.S. patent application Ser. No. 084,395 filed Oct. 12, 1979. However, these schemes are not totally effective and in any case will not reduce cross-coupling between signal channels arising from direct inductive coupling between the playback heads.
Further improvement in chrominance signal-to-noise performance in the two track arrangement may be achieved by frequency-modulating a carrier with the chrominance information in much the same manner as the luminance. However, this degrades the crosstalk or cross-coupling still further, because the luminance and chrominance signals may occupy the same frequency band, and increased coupling occurs between the luminance and chrominance pickup heads.
It is desirable to reduce the effects of coupling between adjacent signal channels where one of the channels contains information modulated onto an FM carrier.